1. Field of the Invention
The invention relates to compositions and methods for reducing cross-linking in the gelatin shell of gelatin capsules by incorporation of free amino acid into the capsule shell and by inclusion of an ester of carboxylic acid either into the capsule filling, and/or into the capsule shell and/or into the lubrication agent, or in combinations thereof. Described are soft gelatin capsules characterized by improved stability as compared with gelatin capsules that do not contain amino acid in the shell and carboxylic acid ester in the filling, shell or in the lubrication agent, or in combinations thereof.
2. Summary of the Related Art
Gelatin capsules, commonly used as a pharmaceutical dosage form or as food supplements, consist of a gelatin shell surrounding a core filled with the composition being delivered. Gelatin capsules and methods of making the same are well known and described in the general pharmacological literature. Gel capsules may be hard (filled with solid or semi-solid fill) formed by two halves joined to create a shell capsule, or a soft (filled with a liquid or semi-liquid fill) formed by a single gelatin shell. Caplets (sometimes also referred to as gel capsules) are modified tablets covered by gelatin shell.
Soft-gelatin capsules are produced by injecting a liquid or a semi-liquid fill between two gelatin strips, either by discontinuous formation or by continuous formation (i.e., rotary-die process). During the manufacturing process, both gelatin gel strips have to be lubricated by a suitable lubricant in order to avoid early sticking to machine parts or to each other. Suitable lubricant agents commonly used in the art include mineral oils (e.g., paraffin oil), synthetic oils (e.g., silicone oil) or vegetable oils (e.g., coconut oil and corn oil).
Liquid or semi-liquid fills for soft-gelatin gel capsules are divided into two basic groups according to their miscibility with water (see, Horn and Jimerson, Capsules, Soft, In: Encyclopedia of Pharmaceutical Technology, Vol. 2, Swabrick and Boylan (Eds), Marcel Dekker, New York and Basel, 269-284, 1990; and Lachmann, Theory and Practice of Industrial Pharmacy, 2nd Ed. Lea and Febiger, Philadelphia, 1986). In a first group are hydrophobic fills capsules (e.g., vegetable and aromatic oils, aromatic and aliphatic hydrocarbons, chlorohydrocarbons, ethers, esters, alcohols and high molecular organic adds), whereas in a second group are hydrophilic fills capsules (e.g., polyethyleneglycols and non-ionogenic surfacial active material—surfactants).
After encapsulation of hydrophobic fills into gelatin gel, excess water leaves the gel and enters the fill. The excess water is almost completely resorbed by the gelatin shell of the capsule during the drying process, until equilibrium with the surroundings is achieved. Conversely, hydrophilic fills are able to absorb a certain amount of water, which can enter the fill after encapsulation. The resorption process is more difficult for these fills and equilibrium achievement is conditioned by the HLB (hydrophilic to lipophilic balance) value and absorption hysteresis curve for gelatin shells of concrete composition (York, J. Pharm. Pharmacol. 33:269 (1981)).
A third group of compounds and compositions, which are not suitable for incorporation in a gelatin capsule includes: water and aqueous solutions, low molecular weight and water-soluble volatile organic substances (e.g., organic acids, alcohols, ketones, aldehydes, amines and esters). Besides their principle unsuitability for encapsulation due to dissolution-related impairment of the gelatin gel, some of these compounds can have deteriorating effect on the gelatin shell even if they are present in trace amounts. These compounds have been found to promote gelatin cross-linking (i.e., the formation of covalent chemical bonds across the polypeptide chains of gelatin) in the gelatin shell resulting in an insoluble polymer structure. Cross-linking may also arise during storage of gelatin capsules giving rise to a molecular (net-like) structure of gelatin, which is almost insoluble or poorly dissoluble in water.
It is also known that gelatin cross-linking in the shell of the capsules is promoted by use of certain chemical compounds in the fill of capsules and can arise in the course of aging of the capsules or after stress conditions. The formation of such a molecular (net-like) structure of gelatin results in worsened disintegration profile of the capsules resulting in a delayed release of pharmaceutically active substances contained in the capsule.
Factors Affecting Dissolution Test of Soft Gelatin Capsules with Pharmaceutically Active Substances
The employment of gelatin capsules as an oral delivery means is well known in the pharmaceutical arts. In pharmaceutical applications, soft gelatin capsules are especially suitable for oral administration of lipophilic active substances. However, once the cross-linking of the gelatin occurs, the gelatin shell becomes less soluble in an aqueous medium, especially in an acidified water medium. The cross-linking delays the disintegration of the gelatin shell, which subsequently delays the dissolution of the inner content of the capsule as compared with a similar capsule not exposed to long time storage or stress-conditions which promote cross-linking.
Therefore, it is necessary when the gelatin capsule contains a component which promotes cross-linking in the gelatin shell to prepare a formulation which will not induce delayed disintegration and/or delayed dissolution of the inner content of the capsule following storage or after exposure to stress conditions.
The shells of both hard and soft gelatin capsules are susceptible to cross-linking. Cross-linking has been demonstrated by a prolongation of the dissolution time and release of drug substance. The delay is attributed to only partial dissolution of the gelatin shell (in case of soft gelatin capsules, the dissolved part is the outer layer of shell). In some instances, the inner layer of the gelatin shell forms a thin film, called a pellicle, which remains intact and envelopes the inner volume of the capsules. This effect is described by Carstensen and Rhodes (Drug Dev. Ind. Pharm. 19(20):2709 (1993)) or Bottom, et al., (J. of Pharm. Sci. 86(9):1057 (1997)).
Considering relatively small intensity of mixing in dissolution apparatus, the rupture of gelatin shell containing the pellicle is worsened and delayed as well and it is the cause of high variability of results of the dissolution test.
There are presently two basic methods described in the literature addressing the dissolution problems of soft gelatin capsules. These methods include (a) demonstrating that the altered dissolution profiles obtained from cross-linked gelatin capsules is a laboratory phenomenon by utilizing in vivo bioequivalence and/or clinical studies which attempt to demonstrate that actual biological availability of test agents are not negatively affected by the cross-linking; and (b) elimination of the causes of cross-linking, namely (i) elimination of physical conditions, which promote the cross-linking, (ii) elimination of substances, which promote the cross-linking (cross-linking promoters), (iii) addition of cross-linking inhibitors (where the most effective action is concurrent combination of these precautions).
In Vivo Studies with Capsules Affected with Cross-Linking
Although the changes of bioavailability of a drug during the shelf-life are always accompanied with the changes in dissolution characteristics and/or are even indicated by them in advance, it is not true vice versa—i.e., the changes in dissolution characteristics do not always necessarily indicate the deterioration of bioavailability.
Chafetz, et al., J. Pharm. Sci. 73:1186 (1984) showed a typical example in which the pellicle formed by gelatin cross-linking did not rupture in vitro, but always disintegrated in the patient's stomach. Similar observations showing that gelatin cross-linking has greater impact on the result of the in vitro dissolution test rather than on the in vivo bioavailability of a drug have been reported. (See e.g., Dey, et at, Pharm. Res. 10(9):1295 (1993), Digenis, et. al., J. Pharm. Sci. 83(7):915 (1994), or Murthy, et al., Pharm. Tech. 13:(6):53 (1989)). Special importance among these studies belongs to the work of Digenis, et al.
While it appears that gelatin cross-linking has far more greater impact on the results of in vitro dissolution tests rather than on in vivo bioavailability, it is still prudent to utilize such formulations of the inner fillings of the gelatin capsules which minimize cross-linking in the gelatin shell and which therefore minimize the effects of time or stress conditions on the dissolution profile of the gelatin capsules. This may be especially important in a case of accelerated stability studies in which the capsules are stressed with enhanced temperature and relative humidity. The reason is that the results of accelerated stability studies are often used for drug registration purposes, and that the dissolution test is often required as one of the stability-indicating methods.
Elimination of the Causes of Cross-Linking
Cross-linking of the gelatin capsules may be promoted or accelerated by physical conditions or chemical substances.
Physical conditions which promote or accelerate cross-linking include: (a) the combination of elevated temperature and humidity (Murthy et al., Pharm. Tech. 72 (1989)), (b) heating (critical temperature is between 37-40° C.) (Hakata et al., Chem. & Pharm. Bull. 42(7):1496 (1994)), (c) dehydration as a result of heat treatment (Welz and Ofner, J. Pharm. Sci. 81(1):85 (1992)), (d) speed of drying (Reich et al., Pharm. Industry 57 (1995)), or (e) UV radiation.
Chemical substances which activate, facilitate or force cross-linking include among others glucose, aldehydes (glutaraldehyde, formaldehyde, glyceraldehyde), peroxides (hydrogenperoxide), epoxides (1,3-butadiene diepoxide), benzene, sulfonic acid, or guanidine hydrochloride. Metal salts react with gelatin also and thus by complexation with the carboxylate groups (chromium salts, zirconium, aluminium). Cross-linking reaction of gelatin is intentionally used in photographic industry for hardening of gelatin gels. A great deal of information about this subject is contained in books (Clark, R. C. and Courts, A., “The Chemical Reactivity of Gelatin” in The Science and Technology of Gelatin, (ed.) A. G. Ward and A. Courts, Academic Press, (1977); Chonan, Y. et at, “The effects of Chemical Modification on the Physical Properties of Gelatin” in Photographic Binders, 2nd Edition, (ed.) H-Irie et al., Published by the Research Group of Photographic Binders in Japan (1990)).
Cross-linking of the gelatin in gelatin capsules is inhibited or reduced by chemical substances, which include among others: aminoacids (glycine, lysine), carboxylic acids (citric acid), semicarbazide, hydroxylamine hydrochloride, piperazide, pyridine, pentamethylene imine, glycerine, or p-aminobenzoic acid.
It is known that gelatin cross-linking in gelatin capsules can be reduced or avoided if the amount of amino-groups available along the molecular chain of the gelatin is reduced, either by masking through covalent bonds with suitable masking agents or by employment of competitive agents containing an abundance of free amino-groups.
A representative masking agent often used is succinic acid, because the two carboxyl groups of this organic acid enable both the reaction of one carboxyl group with an accessible amino-group on the molecular chain of the gelatin and, concurrently providing steric prevention of access of the cross-linking agent. The process where the gelatin is modified by covalent binding of its molecules with succinic acid is called succination (succinization). A disadvantage of this approach results from the fact that the gelatin gels prepared from a modified (succinated) gelatin are characterized by high permeation and as such they are inappropriate for encapsulation of fillings which contain ethanol, propylene glycol or other volatile or migratable ingredients. Besides this, succinated gelatins are still susceptible to cross-linking when complexation agents such as metal salts are the cross-linking promoters.
Free monomeric amino acid is often used as a competition agent, for example glycine or lysine, whose free amino group competes with accessible amino groups on the molecular chain of the gelatin during its reaction with cross-linking agent.
Influence of Glycine on Cross-Linking
The inhibitory effect of glycine on cross-linking is more intensive when free and accessible carboxyl groups are concurrently present. See, for example, Adesunloye and Stach (Drug. Dev. Industrial Pharmacy, 1998).
It appears that glycine prevents cross-linking in both alcohol and non alcoholic formulations for gelatin capsules (unpublished observations of inventors). Glycine is currently used in pharmaceutical preparations, where dimethyl isosorbide is used as a solvent. A glycine concentration of around 0.5% is used for prevention of cross-linking.
Pharmaceutical Acceptance of Glycine Usage in the Shell of Soft Gelatin Capsules—Practical Feasibility
There are no limitations for the use of glycine in capsules as mentioned in the list of adjuvants permitted by United States Food and Drug Administration's (FDA) Inactive Ingredient Guide (IIG) from year 1996. However, from the specifications for tablets, oral powders, food products or beverages, the following supporting data can be obtained:
ConcentrationFood and drinks (CFR Ch. I (Apr. 1, 2000Edition))auxiliary masking substance (glycine)0.2% in final drinkPharmaceuticals (IIG)Intramuscular injections(limits unpublished)freeze-dry powder for intramuscular(limits unpublished)injection solutionCapsules, oral(limit unpublished)Powder for reconstitution solution, oral(2.1%)Oral solution(limits unpublished)Oral tablets  (8.X mg-163.31 mg)Oral tablets (immediate/comp. release),(100 mg-200 mg)uncoatedRectal solution(limits unpublished)Powder for reconstitution of solution,(limits unpublished)subcutaneous
The patent literature discloses glycine contents between 0.1 to 25.0%, most preferably of 0.2 to 5%, related to a fresh gelatin gel. The glycine concentrations presented below describe levels reported for use in some common dosage forms. Amounts in common use include: 1.08 mg per 1 capsule (size 5 oval), 2.52 mg per 1 capsule (size 11 oblong), and 3.94 mg per 1 capsule (size 20 oblongs).
It is also known, that the effect of competition agents (free amino acid containing moieties) can be further intensified (potentiated) by the concurrent use of a substance with a free carboxyl group, generally an organic acid. For organic acids, the monomeric lower carboxyl acid is often used, as for example citric acid. However, this approach has so far been used only in hard gelatin capsules, containing powdered or semi-solid dry fills. It has not been possible to apply this method to soft gelatin capsules, where the dominant lipophilic character of fill components did not allow for the use of lower monomeric organic acids, which are soluble in aqueous and hydrophilic media and insoluble in oily or lipophilic media.
In soft gelatin capsules production, a combination of the above mentioned approaches is used so that the succination of polymeric gelatin chain is performed by reaction with succinic acid anhydride. Concurrently, glycine or lysine is added as a free amino-acid into the gel composition.
However, to date several disadvantages have accompanied this approach. The first drawback comes from the fact that the product formed as a result of the reaction of gelatin with succinic acid anhydride has not been recognized as an acceptable auxiliary pharmaceutical compound (excipients).
The second drawback is caused by the high permeability of gelatin gels prepared from succinated gelatin. This feature makes them unsuitable for encapsulation of fillings containing ethanol, propylene glycol or other volatile or migratable components.